Field of the Invention
This invention relates to isolation technology and more particularly to providing isolation between systems having different voltage domains.
Description of the Related Art
In a typical application, an electromechanical system provides one or more signals for monitoring and/or receives one or more signals for controlling the electromechanical system. During normal operation, a large DC or transient voltage difference may exist between the domain of the electromechanical system and the domain of the monitor or control system, thus requiring an isolation barrier between the electromechanical system and the monitor or control system. For example, one domain may be grounded at a voltage that is switching with respect to earth ground by tens, hundreds, or thousands of volts while the other domain has a 3 V or 5 V voltage swing. Accordingly, an intermediate system includes isolation that prevents damaging currents from flowing between the electromechanical system and the monitor or control system. Although the isolation prevents the electromechanical system from being coupled to the monitor or control system by a direct conduction path, an isolation channel allows communication between the two systems.
Opto-isolation is a technique used to provide the desired isolation. Referring to FIG. 1, an exemplary programmable logic control (PLC) application uses an opto-isolator to provide isolation between the exemplary electromechanical system, which has a voltage domain of 24 V, and driver 107 of a monitor system having a voltage domain of typically 5 V, but may vary, depending on the application. In response to a current being supplied through node 101, light emitting diode (LED) 103 emits light that is received by photo transistor 105, which generates a signal that turns on driver 107. Driver 107 may be coupled to a microcontroller unit of the monitor system.
One shortcoming of the opto-isolators of FIG. 1 is that the output of LED 103 is dependent on the strength of the current through node 101. That is, the stronger the current through node 101, the stronger LED 103 is driven, and the stronger the output of LED 103, which may result in undesirable switching characteristics. In addition, opto-isolators may be susceptible to common mode input transients, requiring an external resistor 111 to overdrive LED 103 to keep LED 103 on when a common mode transient occurs during an output high state. However, the use of external resistor 111 reduces efficiency during operation when the control current is turned off via control transistor 112 due to power dissipation through external resistor 111.
Thus, it would be desirable to provide improved isolation technology with greater immunity to input common mode transients and improved operating efficiency.